Hallucinations often appear during illness because the brain’s normal way of processing information gets disrupted. When someone is sick—whether from a high fever, infection, injury, or certain mental health conditions—the brain can start to misinterpret signals or create sensory experiences that aren’t actually there.
One common reason is that illnesses like infections or fevers can affect the brain directly. For example, a very high fever can cause confusion and make the brain send false signals, leading to hallucinations. Similarly, infections in the brain or damage caused by injury can interfere with how neurons communicate, causing unusual perceptions such as seeing things that aren’t present.
Certain physical conditions like Parkinson’s disease or Alzheimer’s disease also sometimes cause hallucinations because they change how parts of the brain work. Toxic substances like lead or mercury poisoning disrupt normal brain function too and may trigger hallucinations.
Mental health disorders are another important factor. Conditions such as schizophrenia and borderline personality disorder often include hallucinations as symptoms due to changes in how the brain processes reality. Stressful events like trauma or bereavement might also make some people more prone to experiencing hallucinations.
Sleep problems play a role as well. Lack of sleep can confuse the mind enough to produce hallucinatory experiences temporarily. There are even specific types of harmless hallucinations that happen when waking up from sleep (called hypnopompic hallucinations), which occur because parts of dreaming activity overlap with waking consciousness.
Drugs—both recreational and prescribed—can induce hallucinations by altering chemical balances in the brain. Some drugs used medically interact with receptors involved in perception and mood regulation; others taken recreationally have strong effects on sensory processing.
In short, when illness affects either the physical structure of the brain or its chemical environment—or when mental health is impacted—the delicate balance needed for accurate perception breaks down. This disruption causes people to experience sights, sounds, smells, tastes, or sensations that seem real but come purely from altered neural activity rather than external reality.
